The technological advances made in semiconductor processing have enhanced soldering of surface mounted electronic components to conductor pads on etched circuit boards. These circuit boards are generally known as printed circuit boards without regard to the method of forming the circuit traces on the insulating substrate. The economies of scale resulting from this advance have led to the predominance of printed circuit boards for surface mounted devices. Numerous different components and devices (collectively "components") can be surface mounted.
By placing test pads and contacts at specific electrical and/or physical board locations, most circuits and components may be tested for quality control by automated test equipment. For various reasons, certain components (including certain surface mounted components) do not lend themselves to direct electrical testing. Thus, the technology presently known is unable to provide for the electrical testing of either the component or the reliability of the solder connection(s) to it, once the soldering assembly is completed. Examples of such components include intrinsic safety components, surge suppressors, gas discharge tubes, certain semiconductors, and myriad other components. Only costly visual inspection of the connections to these components can be performed to ensure that they are properly soldered.
Occasionally, component to solder pad connections that remain unsoldered may be difficult or impossible to detect. This is especially true of the untestable components previously discussed. Often, through partial physical contact of a component lead and its respective solder pad, the component may still function sufficiently well to pass an initial electrical inspection, at least until shock, vibration, oxidation, and/or corrosion degrades the electrical contact. This defect may be difficult to detect since an intermittent electrical connection may be present when the component or circuit functionality is tested. Often, these improper contacts are very difficult to locate and identify, even under repeated visual inspection.
Automatic testing techniques typically test the electrical or electronic functionality of the components. The equipment and methods for practicing automatic testing are well known to the person having ordinary skill in the art. Automatic testing techniques are desirable since they are faster than manual testing techniques, more efficient, generally more reliable, and more cost effective. Automatic continuity/value testing cannot, however, be used with many of the foregoing special components. Often, these components cannot be electrically tested because testing would destroy the component for its intended function. However, it may still be necessary to verify the solder connection of each component lead to ensure that electrical connection is made. Presently, this inspection must be performed visually. A reliable electrical test of the connection would be preferable, especially if performed with conventional automatic test equipment and testing processes. Automated test of these special components cannot be performed at present since there is no mechanism that can test the solder connection of each lead of these components in an automated manner.
The use of test pads connected to component leads is well known. Such test pad connections provide an electrical connection point for the known automated means for testing electrical connections between various devices and for testing the component functionality. However, there is no automated mechanism directed to testing for the presence or the mechanical stability of the solder joints of individual component leads to the circuit board. When the component can be automatically tested, an unsoldered lead may go undetected if it does not affect the electrical functionality of the tested component(s) at the time of the test.
U.S. patent application Ser. No. 08/451,954, filed May 26, 1995, assigned to the assignee of the present invention, discloses a system and method for detecting unsoldered thru-hole component connections. The teaching of U.S. patent application Ser. No. 08/451,954 is hereby incorporated herein in its entirety.
In Ser. No. 08/451,954, a peripheral land area is positioned very closely circumjacent the land area of a thru-hole on one or both sides of a PC board, through which thru-hole a component lead is inserted. The circumjacent land area(s) may be connected to one or more test pads. The separation between the thru-hole land area and the circumjacent land area forms an isolation gap which is very narrow; it is easily bridged by flowing solder over the separation. Thus, application of solder to secure the components to the thru-hole land area of the PC board effectively bridges, by surface tension of the molten solder, the narrow space between the thru-hole land area and the circumjacent land area connected to the test pad. When the soldering process is completed, measurement of electrical conductivity between the test pad(s) indicates that the component lead is properly soldered on both sides of the board. No portion of the component lead is ordinarily used in bridging the gap.
U.S. Pat. No. 4,091,529 to Zaleckas illustrates a bifurcated component connect lead area on a printed circuit board, joined by the circuit board trace at a location spaced apart from the connect land area. The document does not address automated testing of unsoldered component leads.
U.S. Pat. No. 5,308,928 to Parla et al discloses a method and structure for manufacturing printed circuit board traces in which multiple land areas are formed at open areas of the board and in such close proximity to one another that an intentional interconnect can be formed between many of them by solder bridging. A multiplicity of such interconnect areas enables manufacture of a minimal number of printed circuit boards to each meet the multiple connection possibilities of multiply functional printed circuit boards. The document does not address the problem of defectively soldered component leads or automated testing of unsoldered component leads.
Accordingly, where the component is not susceptible of direct electrical testing there exists no technique for automatically detecting unsoldered component leads. Only a manually performed visual check of each such solder connection can detect such defective solder joints.